1. Field of the Invention
The present invention relates to a laser driving device, an optical scanning device, and an image forming apparatus, and more specifically relates to a laser driving device including a semiconductor laser driving circuit that controls a light output from, for example, a light source of a laser printer, an optical disk device, a digital copying machine, an optical communications apparatus, and an optical scanning device including the laser driving device, and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
In an electrophotography image forming apparatus, an optical scanning device is widely used that includes a laser driving device including a semiconductor laser serving as a light source and a driver driving the semiconductor laser. The optical scanning device generally adopts a method in which a photosensitive element is rotated and scanned with laser light deflected by a deflecting unit, such as a polygon mirror, provided so as to form a static latent image on the photosensitive element in an axial direction of the photosensitive element.
A typical driver for driving a semiconductor laser has both an adjustment function that adjusts the light amount of a semiconductor laser, a semiconductor laser array, or a vertical cavity surface emitting laser (VCSEL) and a function that modulates the laser with high speed. For example, in a color image forming apparatus, generally, an optimum light amount is selected according to efficiency of photosensitive elements of respective colors and an optical system, and a light source is on-off modulated with high speed at the light amount. Refer to the following examples of related art: Japanese Patent Application Laid-open No. S63-197037; Japanese Patent Application Laid-open No. H5-063283; Japanese Patent Application Laid-open No. 2004-128342; Japanese Patent Application Laid-open No. 2008-233115; and Japanese Patent Application Laid-open No. 2008-292956.
If an optimum light amount is to be selected by a semiconductor laser driver alone according to efficiency of photosensitive elements of respective colors and an optical system, light amount adjustment needs to be done over a wide range. For example, if efficiency of the optical system has a range from a minimum to a maximum that is twice the minimum while sensitivity variation of the photosensitive elements has a range from a minimum to a maximum that is twice the minimum, light adjustment needs to be done over a range from a minimum to a maximum that is four times the minimum. For example, if the maximum output power of a semiconductor laser is 10 mW, a dynamic range is needed from 2.5 mW to 10 mW, i.e., the maximum is four times the minimum in the range. As far as such range is concerned, the semiconductor laser driver can sufficiently fulfill the range with the dynamic range of the output current of the driver.
However, such a laser driving device has the following problem. When the driving current of a semiconductor laser fluctuates, i.e., a light output fluctuates, a laser characteristic, particularly, a droop characteristic fluctuates, causing a light waveform to fluctuate. Accordingly, density of images fluctuates when the images are formed by using laser light having different waveforms. As a result, images having stable uniform density cannot be achieved.